This invention relates generally to creep testing fixtures and methods and, more particularly, to test fixtures and methods for measuring creep and creep recovery in tensile test specimens under a variety of environmental conditions.
The characterization of creep, or time-dependent strain, is an important part of the development of polymers and polymer-matrix composite materials. The measurement of creep recovery, or strain decrease after load removal, is also an important part of composite material development. This is because polymers and polymer composites used on ground vehicles must have adequate creep resistance to maintain their dimensional integrity throughout their anticipated road life.
The rate at which a material creeps is a function of stress, temperature and environment. In addition, polymers and polymer composites are often subject to significant specimen-to-specimen variation in creep resistance, thus necessitating the testing of multiple specimens under identical environmental conditions to assure the statistical significance of results.
Currently, creep testing is done on stationary dead-weight creep machines. These dead weight creep machines are large and expensive. In addition, it is seldom practical to use such devices in the field.
A previous test fixture disclosed in U.S. Pat. No. 5,798,463, which issued Aug. 25, 1998, discloses a constant stress/constant strain testing fixture including first and second mounting grips for holding opposite ends of a test specimen. A frame supports the first and second mounting grips for relative reciprocal movement toward and away from each other. The frame comprises a pivoted lever arm frame structure that includes a compression column connected between first and second generally parallel lever arms. The first and second mounting grips are supported on the first and second lever arms, respectively. A compression spring is connected between the first and second mounting grips. More specifically, the compression spring is connected at one end to the first lever arm and at a second end to the second lever arm. The mounting grips are supported between the lever arms at respective points along the lever arms so that a test specimen held between the grips is positioned between and generally parallel to the compression column and the compression spring. This configuration subjects a test specimen held between the mounting grips to tensile stress in response to the axial outward force that the compression spring applies to the mounting grips.
The above constant stress/constant strain fixture was designed to apply stress to a test specimen as the specimen was being exposed to elevated temperatures and/or environmental fluids. Such testing was necessary in view of well-documented observations that environmental agents attack certain materials, such as polymer-based composites, more aggressively when those materials are under stress than when the materials are in an unloaded state. Since all structural applications of these materials include exposure to varying elevated stress levels it was imperative that the behavior of these materials be evaluated using such constant stress/constant strain devices. The device disclosed in U.S. Pat. No. 5,798,463 includes parallel lever arms and a relatively stiff, high-rate compression spring (defined as exerting a relatively large amount of force per unit of deflection) located between the lever arms to subject the test specimen to stress. In tests using these fixtures a test specimen was exposed for a prescribed time to environmental agents while under stress and was then tested to determine to what extent its residual physical/mechanical properties had decayed. While the constant stress/constant strain fixture is completely adequate for its designed purpose, it suffers from a significant shortcoming: that a specimen tested in the constant stress/constant strain fixture will respond to exposure to environmental agents by undergoing stress relaxation due to axial stretching. This, in turn, allows the compression spring to expand and consequently reduces the amount of stress the compression spring applies to the test specimen.
What is needed is a conceptually new creep-testing fixture designed around both the characteristics of spring loaded stressing fixtures and the creep characteristics of structural materials. Based on experience with structural applications of polymers and polymer-based composites, a creep strain boundary condition of 0.5% creep in a 3000 hour creep test has been established. A test specimen material under any combination of stress and environmental agents that exhibits greater than 0.5% creep strain in a 3000 hour creep test is unacceptable and would not be useable in applications under those conditions. With this boundary condition as the basis for selecting material to be used in anticipated applications, the design of a creep fixture to test such material would need to apply an acceptable level of stress to a test sample of the material while accepting a maximum of 0.5% of the applied creep stress. The test fixture must also be sufficiently compact in geometry to permit under-vehicle road testing.
The invention is a creep-testing fixture for applying a precise amount of constant tensile stress to a creep test specimen to allow for precise measurement of creep in the test specimen. The fixture includes first and second mounting grips for holding opposite ends of a test specimen and a frame supporting the first and second mounting grips for relative reciprocal movement toward and away from each other. A spring is connected between the first and second mounting grips. The frame is configured to subject a test specimen held between the mounting grips to tensile stress in response to force that the spring applies to the frame.
Unlike the prior art of record, the spring of the creep testing fixture is a tensile spring and the frame is configured to subject a test specimen to tensile stress in response to axially inward force that the tensile spring applies to the frame. This limits spring load loss over time by an amount sufficient to allow for accurate tensile creep testing of a test specimen. Tensile springs can be stretched extensively to produce a desired amount of creep stress that is limited only by the spring material yield strength. An equivalent compression spring would be unsuited for use in compact creep testing fixtures because it could only be compressed only a relatively short distance before its coils would come into contact with each other and relieve stress on the test specimen.
The invention also includes a method for applying a precise amount of constant tensile stress to a creep test specimen to allow for precise measurement of creep in the test specimen. According to this method one can applying a precise amount of constant tensile stress to a creep test specimen to allow for precise measurement of creep in the test specimen by connecting opposite ends of the tensile test specimen to the respective first and second mounting grips. Opposite ends of the spring are then connected to respective spring attachment points on the lever arms such that the spring applies a generally constant axial inward load to the lever arms and the lever arms apply a generally constant tensile load to the test specimen.
The invention also includes a method of measuring creep recovery that includes connecting opposite ends of the tensile test specimen to the respective first and second mounting grips then connecting opposite ends of the spring to respective spring attachments points on the lever arms such that the spring applies a generally constant axial inward load to the lever arms and the lever arms apply a generally constant tensile load to the test specimen. The resulting strain exhibited in the test specimen is then measured, the spring is returned to a relaxed condition and any resulting decrease in strain exhibited in the test specimen is measured.